This invention relates to a waveform converter circuit for obtaining at an output thereof a square waveform having the same frequency as a sinusoidal waveform applied to an input of the circuit, the square waveform also having a substantially unity mark/space ratio between two given amplitude levels for a range of peak-to-peak amplitude levels of the sinusoidal.
A waveform converter circuit of the above type has application, for example, in a circuit arrangement for recovering binary data transmitted by frequency shift using two tone signals of different frequencies which correspond, respectively, to binary `1` and `0` bits. Such a circuit arrangement can be used in a videotex terminal to receive binary data, representing display information, which is transmitted as tone signals from a data source to the terminal over a telephone line. Another possible use of the circuit arrangement is to receive binary data, which is supplied in the form of tone signals from an audio cassette tape player, into a home computer or other digitally operable apparatus.
The technique of binary data transmission by frequency shift using two tone signals is well known. Detection of the two tone signals in order to recover the binary `1` and `0` bits may involve, either counting over a period of time the number of pulses occurring in the square waveform at the output of the waveform converter circuit, or measuring the period between successive pulse of this square waveform. It is convenient to use the zero crossings of the square waveform for this counting or measuring. It is therefore important for the satisfactory recovery of the binary data that the square waveform has an accurate unity mark/space ratio, because deviation from this mark/space ratio will introduce variation into the count or measurement,
The square waveform can be produced by amplitude-limiting, following slicing of the input sinusoidal waveform mid-way between positive and negative peaks. Because the peak-to-peak amplitude and also the actual d.c. voltage level of the sinusoidal waveform can vary, it is advantageous to make the slicing level adaptive so that it tends to remain at the mid-way setting automatically. It will be apparent that if the slicing level deviates from the mid-way setting, then deviation from the required unity mark/space ratio will result.
The adaptive control of the slicing level can be achieved by means of a corrective feedback signal derived from the output square waveform, this feedback signal being used to adjust the slicing level relative to the actual d.c. voltage level of the input sinusoidal waveform. This feedback signal usually needs to be filtered to remove any a.c. component at the output square waveform frequency from the feedback signal.
Thus, a specific form of waveform converter circuit of the above type can comprise a differential amplifier having a first (e.g. non-inverting) input connected to receive the input sinusoidal waveform, a second (inverting) input connected to receive a feedback signal which defines a switching level, and an output at which the output square waveform is produced, the circuit also including a low-pass filter connected between the amplifier output and said second input, the output square waveform being fed to the low-pass filter which performs both an integrating and a filtering function, the filtered output signal constituting said feedback signal.
For an integrated circuit implementation of this specific form of waveform converter circuit, for a particular application thereof, consideration has been given to forming the integrating low pass filter as a switched-capacitor filter using ratioed capacitors. However, because of the excessively large capacitor ratios which would be required, firstly to filter a.c. components of relatively low frequency and, secondly, to provide a feedback signal over a dynamic range compatible with relatively large d.c. voltage level and peak-to-peak amplitude ranges of the input sinusoidal waveform, such an implementation has been found to be impracticable.